Developmental psychology is the scientific study of how and why humans grow, change, and adapt across the course of their lives. Originally concerned with infants and children, the field has expanded to include adolescence, adult development, aging, and the entire lifespan. Developmental psychologists aim to explain how thinking, feeling, and behaviors change throughout life. This field examines change across three major dimensions, which are physical development, cognitive development, and social emotional development. Within these three dimensions are a broad range of topics including motor skills, executive functions, moral understanding, language acquisition, social change, personality, emotional development, self-concept, and identity formation.

Developmental psychology explores the influence of both nature and nurture on human development, as well as the processes of change that occur across different contexts over time. Many researchers are interested in the interactions among personal characteristics, the individual's behavior, and environmental factors, including the social context and the built environment. Ongoing debates in regards to developmental psychology include biological essentialism vs. neuroplasticity and stages of development vs. dynamic systems of development. While research in developmental psychology has certain limitations, ongoing studies aim to understand how life stage transitions and biological factors influence human behavior and development.

Developmental psychology involves a range of fields, such as educational psychology, child psychopathology, forensic developmental psychology, child development, cognitive psychology, ecological psychology, and cultural psychology. Influential developmental psychologists from the 20th century include Urie Bronfenbrenner, Erik Erikson, Sigmund Freud, Anna Freud, Jean Piaget, Barbara Rogoff, Esther Thelen, and Lev Vygotsky.

Jean-Jacques Rousseau and John B. Watson are typically cited as providing the foundation for modern developmental psychology. In the mid-18th century, Jean Jacques Rousseau described three stages of development: infants (infancy), puer (childhood) and adolescence in Emile: Or, On Education. Rousseau's ideas were adopted and supported by educators at the time.

Developmental psychology generally focuses on how and why certain changes (cognitive, social, intellectual, personality) occur over time in the course of a human life. Many theorists have made a profound contribution to this area of psychology. One of them is the psychologist Erik Erikson, who created a model of eight phases of psychosocial development. According to his theory, people go through different phases in their lives, each of which has its own developmental crisis that shapes a person's personality and behavior.

In the late 19th century, psychologists familiar with the evolutionary theory of Darwin began seeking an evolutionary description of psychological development; prominent here was the pioneering psychologist G. Stanley Hall, who attempted to correlate ages of childhood with previous ages of humanity. James Mark Baldwin, who wrote essays on topics that included Imitation: A Chapter in the Natural History of Consciousness and Mental Development in the Child and the Race: Methods and Processes, was significantly involved in the theory of developmental psychology. Sigmund Freud, whose concepts were developmental, significantly affected public perceptions.

Sigmund Freud developed a theory that suggested that humans behave as they do because they are constantly seeking pleasure. This process of seeking pleasure changes through stages because people evolve. Each period of seeking pleasure that a person experiences is represented by a stage of psychosexual development. These stages symbolize the process of arriving to become a maturing adult.

The first is the oral stage, which begins at birth and ends around a year and a half of age. During the oral stage, the child finds pleasure in behaviors like sucking or other behaviors with the mouth. The second is the anal stage, from about a year or a year and a half to three years of age. During the anal stage, the child defecates from the anus and is often fascinated with its defecation. This period of development often occurs during the time when the child is being toilet trained. The child becomes interested with feces and urine. Children begin to see themselves as independent from their parents. They begin to desire assertiveness and autonomy.